Recovering purge gas
Revamping ammonia plants

Ammonia ranks as one of the world’s most important base chemicals. All industrially manufactured substances containing nitrogen are based on this compound with the chemical formula NH3. Much of the
ammonia produced globally is used for the manufacture of fertilizers, as much as 85 percent in fact. Other industrial uses include the production of nitric acid and polyamides. Every year, around 146 million tonnes of ammonia is produced, with China and Russia leading the way. “In a highly competitive industry, many operators of ammonia plants are looking for ways to make their processes more efficient,” says Christoph Weise, sales project manager at Linde Engineering Dresden. “We can help them do that by
recovering valuable gases from ammonia synthesis purge gas streams.”

A variety of valuable gases going to waste

The industrial-scale synthesis of ammonia produces a purge gas stream. Depending on the process used by the ammonia manufacturer, a typical purge gas stream contains around 60 percent hydrogen, 20 percent nitrogen, five percent argon, four percent ammonia and ten percent methane, which has a negative impact on the catalytic ammonia synthesis reaction. The diverse range of gases produced is attributable to the feedstock, as Weise explains: “Natural gas is used to produce NH3. Following upstream process steps, it ultimately acts as a source of hydrogen. The required nitrogen comes from the air, which of course contains many other gases. Purge gas therefore contains various gas components which do not contribute to ammonia synthesis.”

Plant to recover hydrogen from the purge gas stream from an ammonia production facility in Moron, Venezuela

Purge gas – more than a source of energy

Purge gas must be continuously removed from the process cycle. In the past, plant operators generally just burn it off as a source of energy. However, Linde experts believe that the constituent gases are too valuable to be disposed of in this way. With its purge gas recovery technologies, Linde Engineering Dresden offers ammonia manufacturers a number of options to recover and re-use components from the gas mix. The company can draw on decades of experience in purge gas recovery: Linde experts have been planning, building, delivering and commissioning these plants since 1958.

Cryogenic plants and membrane plants

“Our portfolio now includes a wide range of reliable technologies which we can adapt to the specific requirements of operators, thus indirectly improving the efficiency of their ammonia operations,” says Weise. “Instead of burning off the purge gas, our solutions are able to separate the ammonia, hydrogen, nitrogen and argon.” The most suitable Linde technology depends on the composition of the purge gas in question and the particular constituent gas or gases the operator wishes to recover. Linde Engineering Dresden differentiates between cryogenic plants and membrane plants. “The choice depends on whether only hydrogen is to be separated from the purge gas or whether argon, nitrogen or ammonia are also targeted,” explains Weise.

Optimal separation of gases

The largest constituent of purge gas, hydrogen can be fed directly back in to the ammonia production process. This makes it particularly valuable for plant operators. The gas can be recovered either by using a membrane process or a cryogenic process. With the first method, the gases are separated through fine pores in a two-stage process. Before the gas mix passes through the membrane, the ammonia has to be separated with a gas scrubber. In the cryogenic process, the gases are separated by means of heat exchangers and a separator in a coldbox. Linde offers two different process options here. Depending on whether only hydrogen is to be separated or whether nitrogen and argon are also required, the process comprises multiple stages and additionally includes rectification columns within the coldbox.

Plant to recover liquid argon from the purge gas stream from an ammonia production facility in Hazira, India

High-purity gas recovery

Ultimately, the ammonia manufacturer’s requirements will determine which of the two technologies is most suitable. “For example, the pressure of hydrogen recovered from the purge gas stream and its level of purity are key decision criteria for manufacturers,” the Linde expert explains. Nitrogen and argon can only be recovered by means of cryogenic separation. The operators obtain these gases with a very high purity level, however, for example up to 99.999 percent in the case of argon. Both gases can either be put to other uses in chemical plants or sold.

Revamping old plants

“A purge gas recovery plant usually pays dividends if ammonia production generates hourly purge streams of around 6,000 standard cubic metres,” says Weise. “Nowadays, a recovery unit is usually directly incorporated into new ammonia plants. However, a revamp may still be a worthwhile investment, even with older plants.” Since Linde’s technologies can be easily integrated into existing plants, the recovery of hydrogen, argon and nitrogen is an interesting avenue enabling ammonia producers to remain competitive in the long term.